Grass mowing machines such as trim mowers may be used for trimming along edges which drop off, such as golf course sand traps or bunkers, and for mowing under overhanging obstructions such as trees or bushes. Both of these situations require one or more cutting units to be as far outboard of the traction vehicle as possible. The cutting units should be capable of reaching under overhanging objects while the operator remains seated, and overhanging a drop off while the traction vehicle remains at a safe distance from the edge. Otherwise, the traction vehicle's weight may cause the edge to collapse, especially in sandy soil conditions. Or the traction vehicle may slide into a sand trap or bunker, get stuck, become high centered, or tip over. A mechanism is needed for offsetting cutting units for a grass mowing machine to trim along edges of features such as sand traps or bunkers, and minimize the risk of the traction vehicle sliding into a sand trap or bunker, getting stuck, etc.
To address these requirements, some grass mowing machines, including both reel and rotary types, include mechanisms for offsetting cutting units to one side of the traction vehicle, so that the cutting units may trim on that side only. Some other grass mowing machines provide mechanisms for offsetting cutting units to either side of the traction vehicle. For example, U.S. Pat. Nos. 6,032,441 and 6,351,929 relate to a triplex trim mower with a pair of laterally adjustable cutting units. The cutting units are supported by a lateral carrier frame which is slidably engaged to a lateral support frame. A hydraulic cylinder permits the carrier frame to be laterally displaced with respect to the traction vehicle frame so that a cutting unit can reach the edge of a bunker or sand trap while the vehicle maintains a distance from that feature. When trimming around an obstacle on a slope with the cutting units shifted to the downhill side, the trim mower may become unstable. The instability may result from all cutting units being shifted by a single carrier. The carrier supports its own weight along with the cutting units, cylinders and lift arms. When the single carrier shifts the cutting units to a downhill side, the center of gravity of the mower is moved into a less favorable position that can cause the mower to tip over. If the cutting units are shifted to one side of the machine using the single carrier frame, the opposite side wheel and tire may roll on uncut turf, resulting in poor cut quality and an unsightly cut pattern.
A mechanism for offsetting cutting units for a grass mowing machine is needed that provides high stability, especially on steep slopes. A mechanism for offsetting cutting units is needed that will not shift the center of gravity significantly as the cutting units are extended outwardly. A mechanism for offsetting cutting units is needed that can reduce the risk of damage to the edges of sand traps and bunkers.
Grass mowing machines, including trim mowers, also may need to raise and lower the cutting units on the outer ends of the lift arms. For example, lift arms may be extended to raise the cutting units to a transport or service position, and retracted to lower cutting units to a mowing position. Examples of grass mowing machines with hydraulic implement lift systems for mower cutting units are shown in U.S. Pat. Nos. 5,297,378, 5,343,680, 6,032,441 and 6,351,929. Commonly, lift arms attached to cutting units that are under or near the centerline of a grass mowing machine move about twenty degrees between the lowered and raised positions, while lift arms attached to the outside cutting units move about seventy degrees.